Photolithographic processes in packaging are described in Microelectronics Packaging Handbook, Pub. Van Nostrand Reinhold, New York, 1989, Tummala et al, eds. on pages 898-903, in Principles of Electronic Packaging, McGraw-Hill Book Company, New York, 1989, Seraphim et al, eds. in Chapter 12, pages 372-393 and in Scientific Encyclopedia, 6th Ed., Vol. II, Pub. Van Nostrand Reinhold Company, New York, 1983, Considine et al, eds., pages 1877-1881, all of which are incorporated herein by reference for use as background.
Photolithography plays a critical role in the art of printed circuit packaging. Photolithography is used to define in a thin film of photoresist those regions either from which copper is to be selectively etched to subtractively form circuitization, or selectively plated to additively form circuitization.
There are two types of photoresist: negative and positive. A negative photoresist is polymerized by exposure, e.g., selective exposure to the particular actinic radiation to which it is sensitive for an adequate period of time. It is then subjected to its developer. The developer solubilizes the areas of the resist which have not been exposed to actinic radiation. The areas of negative photoresist which have been exposed to actinic radiation are hardened by cross-linking and made more resistant to developer, relative to the unexposed regions.
Positive acting resists behave oppositely. Actinic radiation renders the positive acting photoresist more soluble in the developer, and the exposed regions are removed preferentially by a dilute alkaline developer.
Positive acting photoresists are used extensively to fabricate silicon devices, and for subtractive circuitization of printed circuit boards. However, positive photoresists, which are readily developed by dilute aqueous alkaline solutions and stripped by more concentrated aqueous alkaline solutions, perform poorly in high caustic environments and high temperatures.
The negative resists, on the other hand, are used when the circuit lines are provided by additive plating of copper, in areas where copper is desired, i.e., electroless or electroless plus electroplating, rather than by etching of copper away from where it is not desired.
Negative acting photoresists are cross-linked by the action of actinic energy on photoactive agents that form the free radicals or ionic groups necessary to initiate and/or support polymerization. Depending on their composition, commercially available photoresists are sensitive to UV radiation, X-rays, E-beams and so forth. The radiation may be furnished to the resist through a pattern in a mask, such as an emulsion mask or chrome mask, by contact or projection, or a beam of radiation may be rastered.
Negative acting photoresists include an organic resin binder, a photoinitiator/photosensitizer and a reactive monomer. Optionally, negative acting photo-resists also include fillers, for example, organic or inorganic fillers, fire retardants, plasticizers, dyes, flexibilizers, thermal stabilizers and other additives to improve the processing characteristics of the package.
Typical negative photoresist compositions include from 40 to 70% by weight of binder, 10 to 40% by weight of monomer, and 0.5 to 15% by weight of photoinitiator, to total 100% based on the weight of all these components.
An example of such compositions is described in U.S. Pat. No. 4,326,010. (example 1).
In general, negative-working resists are photopolymerizable materials of the type described in U.S. Pat. Nos. 3,469,982, 4,273,857 and U.S. Pat. No. 4,293,635 and the photocrosslinkable species of the type disclosed in U.S. Pat. No. 3,526,504.
Monomers which can be used either alone or in combination with others to form negative acting photoresists include: t-butyl acrylate, 1,5 pentanediol diacrylate, N,N-diethylaminoethyl acrylate, ethylene glycol diacrylate, 1,4-butanediol diacrylate, diethylene glycol diacrylate, hexamethylene glycol diacrylate, 1,3-propanediol diacrylate, decamethylene glycol diacrylate, decamethylene glycol dimethacrylate, 1,4-cyclohexanediol diacrylate, 2,2-dimethylolpropane diacrylate, glycerol diacrylate, tripropylene glycol diacrylate, glycerol triacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyoxyethylated trimethylolpropane triacrylate and trimethacrylate and similar compounds as disclosed in U.S. Pat. No. 3,380,831, 2,2-di-(p-hydroxyphenyl)-propane diacrylate, pentaerythritol tetraacrylate, 2,2-di(p-hydrohyphenyl)-propane dimethacrylate, triethylene glycol diacrylate, polyoxyethyl-2,2-di-( p-hydroxyphenyl)-propane dimethacrylate, di-(3-methacryloxy-2-hydroxypropyl) ether of bisphenol-A, di-(2-methacryloxyethyl) ether of bisphenol-A, di-(3-acryloxy-2-hydroxypropyl) ether of bisphenol-A, di-(2-acryloxyethyl) ether of bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of tetrachloro-bisphenol-A, di-(2-methacryloxyethyl) ether of tetrachloro-bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of tetrabromo-bisphenol-A, di-(2-methacryloxyethyl) ether of tetrabromo-bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of 1,4-butanediol, di-(3-methacryloxy-2-hydroxypropyl) ether of diphenolic acid, triethylene glycol dimethacrylate, polyoxypropyltrimethylol propane triacrylate, ethylene glycol dimethacrylate, butylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, 1,2,4-butanetriol trimethacrylate, 2,2,4-trimethyl-1,3-pentanediol dimethacrylate, pentaerythritol trimethacrylate, 1-phenyl ethylene-1,2-dimethacrylate, pentaerythritol tetramethacrylate, trimethylol propane trimethacrylate, 1,5-pentanediol dimethacrylate, diallyl fumarate, styrene,1,4-benzenediol dimethacrylate, 1,4-diisopropenyl benzene, and 1,3,5-triisopropenyl benzene.
In addition to the monomers mentioned above, the photoresist material can also contain one or more free radical-initiated and polymerizable species with molecular weight of at least about 300. Monomers of this type are an alkylene or a polyalkylene glycol diacrylate and those described in U.S. Pat. No. 2,927,022.
Free radical initiators which can be activated by actinic radiation which are thermally inactive at and below 185 degrees Centigrade include the substituted or unsubstituted polynuclear quinones listed in the following: 9,10-anthraquinone, 1-chloroanthraquinone, 2-chloroanthraquinone, 2-methylanthraquinone, 2-tertbutylanthraquinone, octamethylanthraquinone, 1,4-naphthoquinone, 9,10-phenanthrequinone, 1,2-benzanthraquinone, 2,3-benzanthraquinone, 2-methyl-1,4-naphthone, 2,3-dichloronaphthoquinone, 1,4-dimethylanthraquinone, 2,3-dimethylanthraquinone, 2-phenylanthraquinone, 2,3-diphenylanthraquinone, 2,3-diphenylanthraquinone, sodium salt of anthraquinone alpha-sulfonic acid, 3-chloro-2-methylanthraquinone, retenequinone, 7,8,9,10-tetrahydronaphthacenequinone, and 1,2,3,4-tetrahydrobenz(a)anthracene-7,12-dione.
Other useful photoinitiators, of which some may be thermally active at temperatures lower than 85 degrees C, are described in U.S. Pat. No. 2,760,863.
Dyes of a photoreducible nature and other reducing agents are described in U.S. Pat. Nos. 2,850,445; 2,875,047; 3,097,096; 3,074,974; 3,097,097; and 3,145,104 as well as dyes of the phenazine, oxazine and quinone classes; Micheler's ketone, benzophenone, 2,4,5-triphenylimidazolyl dimers with hydrogen donors, and mixtures thereof as described in U.S. Pat. Nos. 3,427,161; 3,479185 and 3,549,367 can be used as initiators. The cyclohexadienone compounds of U.S. Pat. No. 4,341,860 are also useful as initiators. In addition, sensitizers described in U.S. Pat. No. 4,162,162 in combination with photoinitiators and photoinhibitors are useful.
Polymeric binders which can be used alone, or in combination with polymerizable monomers include the following: polyacrylate and alpha-alkyl polyacrylate esters, i.e. polymethyl methacrylate and polyethyl methacrylate; polyvinyl esters: i.e. polyvinyl acetate, polyvinyl acetate/acrylate, polyvinyl acetate/methacrylate and hydrolyzed polyvinyl acetate; ethylene/vinyl acetate copolymers; polystyrene polymers and copolymers, i.e. with maleic anhydride and esters; vinylidene chloride copolymers, i.e. vinylidene chloride/acrylonitrile; vinylidene chloride/methacrylate and vinylidene chloride/vinyl acetate copolymers; polyvinyl chloride and copolymers, i.e. polyvinyl chloride/acetate; saturated and unsaturated polyurethanes; synthetic rubbers, i.e . butadiene/acrylonitrile, acrylonitrile/butadiene/styrene, methacrylate/acrylonitrile/butadiene/styrene copolymers, 2-chlorobutadiene-1,3 polymers, chlorinated rubber, and styrene/butadiene/styrene, styrene/isoprene/styrene block copolymers; high molecular weight polyethylene oxides of polyglycols having average molecular weight from about 4,000 to 1,000,000; epoxides, i.e. containing acrylate or methacrylate groups; copolyesters; nylons or polyamides, i.e. N-methoxymethyl, polyhexamethylene adipamide; cellulose esters, i.e. cellulose acetate succinate and cellulose acetate butyrate; cellulose ethers, i.e. methyl cellulose, ethyl cellulose and benzyl cellulose; polycarbonates; polyvinyl acetal, i.e. polyvinyl butyral, polyvinyl formal; polyformaldehydes.
In addition to the polymeric binders listed above, particulate thickeners such as described in U.S. Pat. No. 3,754,920 i.e. silicas, clays, alumina, bentonites, kaolnites, and the like can be used.
Where aqueous developing of the photoresist is desirable the binder should contain sufficient acidic or other functionalities to render the composition processable in the aqueous developer. Suitable aqueous-processable binders include those described in U.S. Pat. No. 3,458,311 and in U.S. Pat. No. 4,273,856. Polymers derived from an aminoalkyl acrylate or methacrylate, acidic film-forming comonomer and an alkyl or hydroxyalkyl acrylate such as those described in U.S. Pat. No. 4,293,635 can be included.
Normally a thermal polymerization inhibitor will be present to increase the stability during storage of the photosensitive compositions. Such inhibitors are; p-methoxyphenol, hydroquinone, alkyl and aryl-substituted hydroqinones and quinones, tert-butyl catechol, pyrogallol, copper resinate, naphthylamines, beta-napthol, cuprous chloride, 2,6-di-tert-butyl-p-cresol, phenothiazine, pyridine, nitrobenzene and dinitrobenzene, p-toluequinone and chloranil. Also useful for thermal polymerization inhibitors are the nitroso compositions described in U.S. Pat. No. 4,168,982.
Dyes and pigments may also be added to increase the visibility of the resist image. Any colorant used however, should be transparent to the actinic radiation used.
An example of such photosensitive compositions is described in Table I of U.S. Pat. No. 4,693,959.
In the preparation of these formulations generally inert solvents are employed which are volatile at ordinary pressures. Examples include alcohols and ether alcohols, esters, aromatics, ketones, chlorinated hydrocarbons, aliphatic hydrocarbons, miscellaneous solvents such as dimethylsulfoxide, pyridine, tetrahydrofuran, dioxane, dicyanocyclobutane and 1-methyl-2-oxo-hexamethyleneimine, and mixtures of these solvents in various proportions as may be required to attain solutions. Antiblocking agents to prevent the coatings from adhering to the supporting films can also be included.
With some polymers, it is desirable to add a plasticizer, either solid or liquid, to give flexibility to the film or coating. Suitable plasticizers are described in U.S. Pat. No. 3,658,543. A preferred liquid plasticizer is nolylphenoxypoly(ethyleneoxy)-ethanol. A preferred solid plasticizer is N-ethyl-p-toluenesulfonamide.
Photoimageable compositions are also utilized as solder masks. In such application a photoimageable composition is used by applying the composition to printed circuit board and followed by photolithographic techniques to expose various underlying features on the board while masking others. During the soldering process the solder will deposit onto the exposed underlying components. It is necessary that the solder mask material be formulated such that it can be applied by the appropriate methods, for example curtain coating. Suitable photoimageable compositions including many that use epoxies are described in the following U.S. Pat. Nos. 4,279,985; 4,458,890; 4,351,708; 4,138,255; 4,069,055; 4,250,053; 4,058,401; 4,659,649; 4,544,623; 4,684,671; 4,624,912; 4,175,963; 4,081,276; 4,693,961; and 4,442,197.
More recently an improved cationically photoimageable solder mask is described in U.S. Pat. No. 5,026,624 assigned to the assignee of the present application, disclosure of which is incorporated herein by reference. In fact U.S. Pat. No. 5,026,624 teaches an improved photoimageable cationically polymerizable epoxy based coating material.
In processing negative working resists, unexposed areas of the imaged film are typically removed from the surface of a printed circuit board or substrate by action of a liquid developer in a spray form for a duration of several minutes or less. Depending on the particular type of photoresist composition the liquid developer may be a simple organic solvent, an aqueous solution of an inorganic base, or as described in U.S. Pat. No. 3,475,171, a combination of organic solvent and aqueous base to form a semi-aqueous developer.
Methyl chloroform (MCF, 1,1,1-trichloroethane), and methylene chloride (MC, dichloromethane) are solvents which are widely used in the electronic packaging art and in other arts for developing and removing a number of photoresists which are otherwise resistant to chemical attack.
The highly alkaline electroless copper plating baths used in additive processes provide a harsh environment for photoresist. In general, the more chemically impervious resists are removable in an organic solvent such as methylene chloride. For less demanding chemical environments, aqueous developable photoresists may be adequate. The organically developable resists, however, continue to be used in an electroless copper environment and in the print band and thin film technologies in conjunction with acrylate-based resist such as DuPont's Riston T-168 and solvent processed solder masks such as the DuPont Vacrel 700 and 900 series, environments in which the aqueous resists are vulnerable.
The use of 1,1,1-trichloroethane and methylene chloride is disfavored because of growing environmental concerns over the effect of gaseous halogenated hydrocarbons on the depletion of earth's ozone layer and concerns over introducing suspected carcinogens to the atmosphere. Several countries have set goals for their total elimination. However, there continue to be many manufacturing processes in which use of resists which are aqueously developable simply is not feasible.
The industry therefore continues the search for organic solvents as alternates to 1,1,1-trichloroethane and methylene chloride. The new solvents must meet specific manufacturing and environmental requirements with respect to flammability, toxicity, ability to effect dissolution, shelf-life, waste disposal, ability to recycle, simplicity of composition, and compatibility with a spectrum of resists.
Alternative solvents for stripping solvent based Riston photoresists are also described in Research Disclosures, June 1989 p. 302, published anonymously.
There have been previous attempts reported in the art to provide environmentally friendly alternatives to 1,1,1-trichloroethane and methylene chloride. However, none of the references describe the simple, environmentally acceptable, room temperature developers and strippers described by the commonly assigned, copending U.S. application Ser. No. 07/781,542, filed Oct. 22, 1991, of N. R. Bantu, Anilkumar Bhatt, Ashwinkumar Bhatt, G. W. Jones, J. A. Kotylo, R. J. Owen, K. I. Papathomas, and A. K. Vardya for Photoresist Develop And Strip Solvents and Methods for their Use. This application describes the use of 4-methyl-1,2- dioxolan-2-one (propylene carbonate, methyl ethylene carbonate, 1,2-propylene carbonate) as a developer and as a stripping agent. This material has the structure: ##STR1## Bantu et al describe its use as an alternative to halogenated hydrocarbon developers and strippers for use in developing and stripping acrylate based photoresist such as Riston T-168 and polymethyl methacrylate, and solvent processed solder masks such as the Vacrel 700 and 900 series.
U.S. application Ser. No. 07/781,542 describe developing the radiation-exposed resist in a high boiling solvent selected from the group consisting of propylene carbonate (PC), gamma butyrolactone (BLO) and benzyl alcohol (BA). The process occurs at about 24 to 45 degrees Centigrade for about 0.5-12 minutes and is normally followed by a warm water or alternate low boiling solvents rinse to remove excess developer.
The aforementioned solvents of U.S. application Serial No. 07/781,542 are high boiling solvents, while the common developers of the prior art for developing Riston type photoresists are low boiling solvents. The use of low boiling solvents such as methyl chloroform (MCF), methyl ethyl ketone (MEK), xylenes or mixtures thereof are similar to the methylene chloride stripping process.
By way of contrast high boiling solvents, i.e. n-methyl pyrolidone (NMP), gamma-butyrolactone (BLO), dimethyl sulfoxide (DMSO) and propylene carbonate (PC) must be followed by a rinsing step with compatible solvent or water. Furthermore, in order to obtain dissolution times comparable to those of MC, it is necessary that the temperature during stripping be maintained between 50 degrees Centigrade and 100 degrees Centigrade and conditions. Moreover, brushing is necessary during stripping for product quality and high throughput. However, the combination of brushing with these relatively high temperatures results in the removal of a photoresist product containing solubilized and solid photoresist polymer, as well as monomers, additives, initiators, surfactants, dyes and other components, hereinafter collectively referred to as "photoresist products" and "photoresist solids".
Thus, there is a clear need for a low cost process for the separation and recovery of cyclic alkylene carbonate solvents, as propylene carbonate, from the photoresist materials for recycle and reuse.